1. Field of the Invention
This invention relates to the field of rheology modification agents for use in fluids such as drilling, mining and milling fluids, including but not limited to those used in natural resources exploration, utility horizontal directional boring, and environmental remedial wells.
2. Background Art
The use of rheology modification agents, frequently thixotropic agents, for aqueous fluids has been common practice in a large number of industries. These fluids include, for example, oil field drilling fluids, metal-working fluids, mining fluids, fire control fluids, agricultural organic formulations, water-based paints and coating fluids, stripping fluids, and the like. For each of these, and other, applications, the rheology modification agents serve very specific purposes tailored to the function for which the fluid is being employed. Because a variety of properties are frequently needed for a given fluid, the rheology modification agent is frequently used in conjunction with other types of agents or additives, in order to produce a final fluid suitable to a given application. However, it is generally desirable to reduce the number of such agents or additives as much as possible, in order to facilitate the ease of production and use and therefore to also minimize cost.
Certain inorganic materials, including mineral clays such as bentonite and attapulgite, are commonly used for rheology modification of aqueous fluids. For example, bentonite, which is one of the most popular of the clay materials for use in drilling muds, is thixotropic in combination with water and is also relatively stable to temperature extremes. Furthermore, it imparts resistance to oxidation and durability when exposed to high shear conditions. These clays may be used with other types of agents or densifiers, such as iron oxide or barium sulfate, which enhance the ability of the fluid to resist pressures such as are encountered in subterranean excavations.
Aqueous fluids containing only clays for rheology modification suffer problems, however, including compromise in the presence of polyvalent cations, such as calcium and magnesium. These cations are frequently present in drilling formations and can cause the drilling mud to become so thick at higher temperatures that thinners may be required. Some clay systems also suffer from undesirable reactivity and temperature degradation, and may not be adequately consistent in composition from batch to batch.
Combinations of clays and polymeric materials have been employed in attempts to counter deficiencies in simple clay/water systems and also to extend the clay. Typical extenders useful with bentonite systems include polyacrylamide and polyacrylic acid. However, combining clays with polymeric materials increases the complexity of the composition and, therefore, its cost and/or difficulty of preparation, particularly under field conditions. Also unfortunately, the weaknesses of the extending polymer, such as thermal instability, may then dominate the characteristics of the fluid as a whole.
For some applications it is particularly desirable to use a rheology modification agent which exhibits the ability to “shear-thin”, i.e., to show a rapid or near-instantaneous and predictable viscosity reduction upon application of shear forces, followed by a rapid or near-instantaneous return to original viscosity when the shear forces are removed. Clays alone may effectively thicken a composition, but generally do not offer this shear-thinning capability. While there are newer rheology modification agents that do offer shear-thinning capability, they tend to be much more expensive than clay and may be more difficult to employ under typical field conditions.
It would therefore be highly useful to identify a clay-based rheology modification agent for use with an aqueous fluid which takes advantage of the relatively low-cost thixotropy of clay, yet offers shear-thinning capability; which is not cost-prohibitive for large scale application; and which does not undesirably interfere with other commonly-sought properties of the fluid.